Electroless cobalt plating bath and process

ABSTRACT

ELECTROLESS DEPOSITION OF COBALT FROM AN AQUEOUS ALKALINE BATH ON A PALLADIUM ACTIVATED SUBSTRATE. BATH CONTAINS HYPOPHOSPHITE AS REDUCING AGENT, LINEAR POLYPHOSPHATE ANION AND AMMONIUM HYDROXIDE AS COMPLEXING AGENTS AND SMALL CONTROLLED AMOUNTS OF NICKEL ION. COBALT WILL ALSO DEPOSIT WITHOUT IONS BEING IN BATH BY PROVIDING NICKEL ON SUBSTRATE.

United States Patent O 3,745,639 ELECTROLESS COBALT PLATBNG BATH AND PROCESS Nathan Feidstein, Kendall Park, Nul assignor to RCA Corporation No Drawing. Continuation-impart of application fier. No. 372,750, Oct. 30, 1969. This application Oct. 28, 1971, Ser. No. 193,582

int. Ci. C23c 3/02 U.S. Cl. 117-47 R 5 Claims ABSTRACT OF THE DESCLOSURE Electroless deposition of cobalt from an aqueous alkaline bath on a palladium activated substrate. Bath contains hypophosphite as reducing agent, linear polyphosphate anion and ammonium hydroxide as complexing agents and small controlled amounts of nickel ion. Cobalt will also deposit without nickel ions being in bath by providing nickel strike on substrate.

This application is a continuation-in-part of application Ser. No. 872,750 filed Oct. 30, 1969 and now abandoned.

BACKGROUND OF THE INVENTION It has been well known that a number of metals may be deposited autocatalytically on various substrates, including some which are non-metallic, from a plating bath, without the use of an electric current. This type of process has become known as electroless deposition. Some substrates require pro-treatment as with a catalyst, such as a palladium salt, prior to deposition of the metal. Other substrates themselves exert suificient catalytic action without further preparatory treatment.

Cobalt is one of the metals which has been deposited by electroless techniques. One type of bath which has been used for electroless deposition of cobalt contains (a) a salt that furnishes cobalt ions, (b) a complexing agent for cobalt ions to prevent formation of undesired precipitates, (c) ammonium hydroxide which provides hydroxyl ions to maintain an alkaline pH and also aids in complexing the cobalt ions, and (d) a hypophosphite which serves as a reducing agent for the cobalt compounds. Other reducing agents such as amine boranes have also been tried in these baths.

Most of the baths that have previously been reported for electroless deposition of cobalt have operated at elevated temperatures (about 50 C.). At these temperatures it is difiicult to maintain good control of the temperature and of the composition of the deposit.

Thin metallic films of cobalt or of cobalt-phosphorus alloys are of interest because of their magnetic properties. Some of the applications, such as high density recording systems in which these films may be used, require uniformity of magnetic properties. Cobalt films deposited by previously-known methods are not always satisfactory for such applications.

In comparison with films of other magnetic materials, such as nickel, cobalt deposits are harder and are more readily passivated.

DESCRIPTION OF PREFERRED EMBODIMENT In the baths of the present invention, complexing agents including linear polyphosphate anions contribute to obtaining improved cobalt films. A linear polyphosphate is a salt in which the phosphate anion has the formula (P O An example of a preferred deposition bath is as follows.

3,745,039 Patented July 110, 1973 Example I N34P207-1OHzO g./l 70 COSO .7H O g./l 50 NH OH cone. (approx. 58%) cc./l 7.5 g./l 1.5

In the above bath composition the amount of dimethylamine borane is based on room temperature operation. If the temperature is raised, the amount of borane can be decreased. Also, methylamine borane can be used as the reducing agent, and, since this substance is a stronger reducing agent than the corresponding dimethyl compound, smaller amounts are required. In general, the reducing agent in this example may be a substituted amine borane where the side chain is of low molecular weight such as methyl or ethyl. Other specific examples are monoethyland diethylamine borane. Tertiary butyl amine borane may also be used.

The preferred composition of Example I may be varied as follows:

COSO JH O ..g./l 7.6-50 N21 P O .10H O g./l 2014() NH OH (approx. 58%) cc./l 2.3-30 (CH NHBH g./l 0.23-5

These baths operate at room temperature or above and yield cobalt-boron deposits on a wide variety of metals autocatalytically, including palladium-activated substrates. It has been found that cobalt-boron deposits of the present invention are self-initiating on substrates made of chromiurn, aluminum, molybdenum, tungsten, silver, copper and gold or any combination thereof.

Cobalt-boron deposits made with the baths of the present invention are also suitable for printed circuits and other similar applications. A pattern of the cobalt-boron is deposited on an activated, insulating substrate and a layer of copper or nickel is then deposited electrolytically on the cobalt-boron film.

Sodium hypophosphite may also be used as the reducing agent in the bath of the present invention. A preferred example using this type of reducing agent is as follows:

Example II Na P O JOH O g./l 70 CoSO .7H O g./l 50 NH OH conc. (approx. 58%) cc./l 7.5 Nafl POgH O g./1 26 This bath, like that of Example I, may be used at room temperature, although it has a slower plating rate. The plating rate may be increased by raising the temperature. The cobalt deposits as a cobalt-phosphorus alloy.

It has been found in accordance with the present invention that cobalt-phosphorus baths of the present system which includes a polyphosphate complexing agent, will not deposit spontaneously on insulating substrates (or other non-catalytic surfaces) even after these surfaces have been sensitized and activated in conventional manner with palladium. It has now been unexpectedly found, however, that if even small controlled amounts of nickel ion are present in the baths, the cobalt-phosphorus baths with polyphosphate complexing agent will deposit metal spontaneously on palladium-activated substrates. The cobalt phosphorus will also deposit if a thin strike of nickel is first deposited on the substrate. By thin strike is meant at least about 500 A. This is unexpected since, without nickel being present, the baths will not deposit on palladium which is a stronger catalyst than nickel.

The following is an example of carrying out this feature of the present invention. A glass surface is carefully cleaned, then sensitized by dipping in an aqueous sensitizing solution containing about 30 gms. of SnCl -2H O and 40 cc. concentrated hydrochloric acid per liter. After thorough rinsing, the sensitized surface is activated by treating with an aqueous solution containing 1 gm. per liter of palladium chloride and 1 cc. per liter of concentrated hydrochloric acid. The sensitized and activated sub strate is then utilized to deposit cobalt-phosphorus alloy as described below.

To test the effect of nickel on initiating cobalt deposition, bath were made up to be used at 20 C. and 40 C. as set forth below. The source of cobalt was CoSO -7H O but it may be any soluble cobalt salt. Therefore, the cobalt is expressed in terms of Co++.

For use at For use at 20 C. 40 0.

g NfliPgOqlOHgO, g./l 9O 9O NaHzPOz-Hz g./l 25 25 NH-iOH (58%), ce./l 30 105 Using the above compositions, controlled additions of of NiSO -6H O were made with the following results. In the tables the nickel concentration is expressed as Ni++ since the nickel could be supplied by any soluble nickel salt.

TABLE I Results at 20 C. on Palladium-activated Substrate Approximate time to initi- I Concentration of added nickel as Ni++ ate platin Uniformity in g./l. (minutes of plating None 0. 223 13 Poor. 0. 446 13 Do. 0. 892 4 Acceptable.

1. 34 2 Good. 1. 78 2 D0. 2. 23 1. Do.

I No indication.

TABLE II Results at 40 C. on Palladium-activated Substrate Approximate time to initi- Concentration of added nickel as Ni++ ate platin Uniformity in g./1. (minutes of plating Very poor.

3 Poor. 3 Do. 0. 5 Good. 11.2 10' 0.5 Do.

In order to further test the effects on self-initiation of a cobalt-phosphorus bath containing much smaller amounts of cobalt, the following bath was prepared.

C0++ g /l 1.6 Na P O -10H O g./l 25.0 NaH PO -H O g./l.. 20.0 NH OH (58%) cc./l 8

This bath was tested by adding varying amounts of NISO4'6HZO.

I No plating after 10 minutes.

The various ingredients of these baths other than the nickel salt can be varied within the limits indicated in the previous examples in which no nickel compound was present. As the temperature of the bath rises, the amount of nickel needed to initiate good cobalt-phosphorus deposition decreases. However, even at room temperature, deposition can readily be initiated when the cobalt salt 4 concentration is relatively low, for example, like that of Table III above.

The cobalt can be obtained from soluble salts other than the sulfate and can be used in any amount up to saturation. As the cobalt concentration increases, the amount of nickel needed to initiate deposition decreases.

The composition of Example II may be varied as follows:

CoSO -7H O g./l 7.6-50 Na P O -10H O g./l 20-140 NH OH (approx. 58%) cc./l 2.3-30 NaH PO -H o g./l 5-50 Sodium pyrophosphate is only one example of a compound containing a polyphosphate ion that can be used in the baths of the present invention. Another example is a tripolyphosphate (P O Other metals such as nickel, iron, molybdenum and tungsten can be co-deposited with cobalt using the baths of the present invention, by incorporating ions such as Ni+ Fe+ M005 and W05 Although ammonium hydroxide has been given as an example of an agent which both aids in maintaining an alkaline pH and serves as a complexing agent, amines such as ethylene diamine can be substituted for the ammonium hydroxide.

I claim:

1. An aqueous plating bath for the electroless deposition of cobalt-phosphorus alloy comprising:

co++ g./l 1.610 Na P O -10H O g./l 20-140 NH4OH (58% cc./l 2.3 NaH PO -H O g./l 5-50 Ni++ g./l 0.0s5 2.23

2. A method of depositing a cobalt-phosphorus alloy on a non-catalytic substrate comprising sensitizing said substrate and activating said substrate with palladium and then treating said activated substrate with a solution at about 20 C. comprising:

Co g./l 1.6l0 N34P207 g./1 NH OH (58%) CC./i 2.3-30 NaH Po 'H O g./l 5-50 Ni g./1 0.892-about 2.23

3. A method of depositing a cobalt-phosphorus alloy on a non-catalytic substrate comprising sensitizing said substrate and activating said substrate with palladium and then treating said activated substrate with a solution comabout 40 C. comprising:

Co g./l 1.6-10 Na P O -10H O g./l 20-140 NH OH (58%) cc./l 2.3-30 NaH PO -H O g./l 55O Ni++ g./1

About 0.055about 0.22

4. A method of depositing a cobalt-phosphorus alloy on a non-catalytic substrate comprising sensitizing said substrate and activating said substrate with palladium and then treating said activated substrate with a solution comprising:

Co g./l 1.6-10 Na P O -10H O g./l 20-140 NH OI-I (58%) cc./l 2.3-105 NEtHgPOz'HgO g./.1 5-50 Ni++ g./c 0.0275 to about 2.23

5. A method of depositing a cobalt-phosphorus alloy on a non-catalytic substrate comprising sensitizing said substrate and activating said substrate with palladium, depositing a thin strike of nickel having a thickness of at least about 500 A. on said substrate, and then treating said substrate with a solution comprising:

Co++ g./l 1.6- N21 P O '10H- O g./l -140 NH OH (58%) cc./l 2.3-

NQHgPOg'HzO g./l

References Cited UNITED STATES PATENTS 3,627,545 12/1971 Mallory, Jr. et a1.

117-130 E X 2,532,284 12/1950 Brenner et a1. 117-130 E X 3,423,214 1/1969 Koretzky 106-1 6 3,370,979 2/1968 Schmeckenbecher 106-1 X 2,690,401 9/1954 Gutzeit et a1 117-47 A 2,287,400 3/1958 Eisenberg et a1. 117-130 E 3,666,529 5/1972 Wright et a1 117-50 3,024,134 3/1962 Nixon et a] 117-130 E 3,597,267 8/1971 Mallory, Jr. et a1. 106-1 X 3,607,389 9/1971 Canegallo et a1. 117-130 E X ALFRED L. LEAVITT, Primary Examiner 10 J. R. BATTEN, JR., Assistant Examiner US. Cl. X.R.

106-1; 117-50, 71 R, E, 130 B, R; 204-38 5 

